The present invention relates generally to imaging using cone beam scanning. More specifically, it relates to such three-dimensional (3D) tomographic imaging using a scan pattern to image a region of interest.
In conventional computerized tomography (CT) for both medical and industrial applications, an x-ray fan beam and a linear array detector are used. Two-dimensional (2D) imaging is achieved. While the data set may be complete and image quality is correspondingly high, only a single slice of an object is imaged at a time. When a 3D image is required, a stack of slices approach is employed. Acquiring a 3D data set on 2D slice at a time is inherently slow. Moreover, in medial applications, motion artifacts occur because adjacent slices are not imaged simultaneously. Also, dose utilization is less than optimal because the distance between slices is typically less than the x-ray collimator aperture, resulting in double exposure to many parts of the body. In 2D CT, the scanning path of the source is often a simply circular scan about the object. The linear array detector is fixed relative to the source. (Although it is usual to talk about a scan path of a source relative to the object to be imaged, it is to be appreciated that the object may be rotated or otherwise moved to provide relative motion between the object and the source.)
In a system employing true cone beam geometry for 3D imaging, a cone beam x-ray source and a 2D area detector are used. An object is scanned, preferably over a 360.degree. angular range, either by moving the x-ray source in a scanning circle about the object or by rotating the object while the source remains stationary. In either case, the area detector is fixed relative to the source. The relative movement between the source and object which is to be imaged provides scanning in either case. Compared to the conventional 2D stack of slices approach to achieve 3D imaging, the cone beam geometry has the potential to achieve rapid 3D imaging of both medical and industrial objects with improved dose utilization.
In order to provide a complete set of projection data for accurate imaging of an object of interest or a region of interest in an object, it is necessary to satisfy completeness criteria. These criteria, which are described in detail in Smith, B. D., "Image Reconstruction from Cone-Beam Projections: Necessary and Sufficient Conditions and Reconstruction Methods", IEEE Transactions Medical Imaging, MI-4 (1985) pp. 14-25, hereby incorporated by reference, basically require that any plane passing through the object or region of interest must intersect the scan path at one or more locations.
The completeness criteria are also discussed in Ser. No. 07/725,142, incorporated by reference above and which discards unwanted Radon data while obtaining a complete data set.
The 3D CT imaging generally uses a Radon transform approach. (Radon transforms are also used in 2D CT.) The object is defined in terms of its x-ray attenuation coefficient. The measured cone beam projection data corresponds to a line integral of this function over the radial direction from the radiation source to a particular detector element within the detector array. The 3D Radon transform of an object at a point is given by the area integral of the x-ray attenuation coefficient over the plane passing through the point, which plane is perpendicular to the line from the origin to the particular point. If parallel beams of x-rays are applied to the object which is to be imaged, line integrals of the detector data are equal to the Radon transform of the object. However, obtaining the Radon transform is significantly more complex where a cone beam of x-ray or other imaging energy is applied to the object. In that case, obtaining the Radon transform, also called Radon data, is significantly more difficult. Once Radon data is obtained, an inverse Radon transformation is used to convert the Radon data into a reconstructed image which is then displayed.
The U.S. Pat. No. 5,257,183 incorporated by reference above, discloses a technique for calculating the radial derivative of Radon data from cone beam data. The incorporated by reference application Ser. No. 07/631,818 discloses a technique for inverting the Radon data to obtain the reconstructed image of the object which is being viewed. In order to perform the Radon inversion, Radon data (as opposed to derivatives of Radon data) is required (except where using those few techniques which perform Radon inversion using derivative data) and the Radon data should reside on polar grids on a number of predetermined vertical planes containing the Z axis as the common axis. These requirements arise because the first part of the Radon inversion process is a two dimensional (2D) CT image reconstruction on each vertical plane, which takes input data in the form of Radon data at equally spaced angle .theta. and equally spaced detector spacings s. However, the technique of the referenced U.S. Pat. No. 5,257,183 initially produces radial derivatives of the Radon data, instead of Radon data itself, and the derivative data is generated on a spherical shell having as its diameter a line segment so connecting a source position s and an origin o (instead of being generated on the points of the polar grids). The U.S. Pat. No. 5,257,183 further describes techniques for converting from the radial derivative of Radon data to Radon data itself and to obtain the Radon data on the polar grid points by use of the Radon data relative to the spherical shell, often called the Radon shell. However, the calculation of Radon data over the spherical Radon shell requires a relatively large amount of processing or computational power. Further, using that Radon data to provide Radon data at the points on the polar grid of the vertical planes requires relatively complex techniques which, in effect, involve interpolation of different data points on the Radon shell over the shell. This three-dimensional (3D) interpolation is relatively complex and accordingly requires large amounts of computational power.
The previous application Ser. No. 08/100,818, incorporated by reference above, provides for the simplification of the generation of Radon data.
The last three mentioned applications generally provide techniques allowing reconstruction of images using projection data. However, these and other reconstruction techniques may have difficulties in imaging objects and regions which have a rather long, wide, or tall dimension. If the height, width, or length of an object or region is great, it may be impractical or difficult to obtain a detector array with sufficient height or width to obtain projection data from the object or region of interest. Generally, the detector must have a height and width at least somewhat greater than the height and width of the object or region of interest. Otherwise, some x-ray data would be missing. Also, since some x-rays have passed through portions of an object which are not in the region of interest (where the region of interest is only part of an object), the cone beam data collected would not exclusively represent data from such a region of interest.
Some techniques use approximation and other procedures to compensate when the data will be incomplete due to the object or region of interest extending further in one dimension than the detector can image.
U.S. Pat. No. 5,032,990 of Eberhard and Tam, issued Jul. 16, 1991, entitled "TRANSLATE ROTATE SCANNING METHOD FOR X-RAY IMAGING", assigned on its face to the assignee of the present application, and hereby incorporated by reference, discloses a technique for two-dimensional imaging of an object which is so wide that a linear array detector is not wide enough to span the object or part which is to be viewed.
U.S. Pat. No. 5,187,659, in the name of Eberhard and Tam, entitled "CONE BEAM SCANNING TRAJECTORIES FOR THREE-DIMENSIONAL COMPUTERIZED TOMOGRAPHY DATA ACQUISITION WHERE OBJECT IS LARGER THAN THE FIELD OF VIEW", assigned to the assignee of the present application, and hereby incorporated by reference discloses a technique for avoiding corrupted data when performing 3D CT on an object larger than the field of view. (No representation is made or intended that this referenced application is necessarily prior art to the present application.)
U.S. patent application Ser. No. 07/998,330, filed Dec. 30, 1992, in the name of Eberhard, Tam and Hedengren, entitled "THREE DIMENSIONAL COMPUTERIZED TOMOGRAPHY SCANNING CONFIGURATION FOR IMAGING LARGE OBJECTS WITH SMALLER AREA DETECTORS", assigned to the assignee of the present application, and hereby incorporated by reference, discloses the imaging of large-objects using a relatively small detector by moving the detector relative to the source of imaging energy.
Ser. No. 08/131,180, incorporated by reference above, discloses a technique for collecting complete cone beam data of a long or tall object using a short or small detector moved in a helical path.
Although the above and other techniques have been useful, the imaging of a relatively large region of interest portion of an object with a relatively small detector has required processing of data for outside the region of interest and/or scan paths which have abrupt shifts in direction. Since the object being imaged may be a patient, such abrupt shifts in scan direction are undesirable if the patient must be moved for the scanning. Even if the source is moved to obtain an abrupt shift, this is less than desirable.